Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition

Lithofacies and biofacies changes through the Ordovician and Silurian transition on the Yangtze platform are coincident with stepwise mass extinctions, recovery events, global sea-level changes, and regional palaeogeographic configurations. Facies patterns through three intervals, the late-mid Ashgill, Hirnantian, and early Rhuddanian, indicate that black shale occupied most of the Yangtze platform region during the late–mid Ashgill and early Rhuddanian, while the Hirnantian possesses more diverse facies types. The black shale was replaced by carbonate facies during the Hirnantian coincident with a global sea-level low stand. Biofacies changes from the late–mid Ashgill graptolitic to the Hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies, the Manosia facies, in the Yangtze region. However, the biofacies change was abrupt from the Hirnantian to the early Rhuddanian when graptolitic facies occupied most areas on the Yangtze platform leaving only a very narrow, limited belt of shelly facies along the southern margin of the platform in northern Guizhou. The early Rhuddanian unified biofacies pattern is a result of a global sea-level rise, which may be synchronized with a recovery bioevent of graptolites after Hirnantian mass extinction.